Even on a night which is too dark for diurnal vision, invisible infrared light is richly provided by the stars. Human vision cannot utilize this infrared night time light from the stars because the so-called near-infrared portion of the spectrum is invisible for humans. A night vision device of the light amplification type can provide a visible image replicating the night time scene. Such night vision devices generally include an objective lens which focuses invisible infrared light from the night time scene onto the transparent light-receiving face of an I.sup.2 T. At its opposite image-face, the image intensifier tube provides an image in visible yellow-green phosphorescent light, which is then presented to a user of the device via an eye piece lens.
A contemporary night vision device will generally use an I.sup.2 T with a photocathode behind the light-receiving face of the tube. The photocathode is responsive to photons of infrared light to liberate photoelectrons. These photoelectrons are moved by a prevailing electrostatic field to a microchannel plate having a great multitude of dynodes, or microchannels, with an interior surface substantially defined by a material having a high coefficient of secondary electron emissivity. The photoelectrons entering the microchannels cause a cascade of secondary emission electrons to move along the microchannels so that a spatial output pattern of electrons which replicates an input pattern, and at a considerably higher electron density than the input pattern results. This pattern of electrons is moved from the microchannel plate to a phosphorescent screen by another electrostatic field to produce a visible image.
A power supply for the I.sup.2 T provides the electrostatic field potentials referred to above, and also provides a field and current flow to the microchannel plate(s). Conventional night vision devices (i.e., since the 1970's and to the present day) provide automatic brightness control (ABC), and bright source protection (BSP). The former function maintains the brightness of the image provided to the user substantially constant despite changes in the brightness (in infrared and the near-infrared portion of the spectrum) of the scene being viewed. BSP prevents the photocathode from being damaged by an excessively high current level in the event that a bright source, such as a flare or fire, comes into the field of view.
The ABC function is accomplished by providing a regulator circuit monitoring the output current from the phosphorescent screen (See FIG. 9). When this current exceeds a certain threshold, the field voltage level across the opposite faces of the microchannel plate(s) is decreased to reduce the gain of the microchannel plate(s), as is graphically depicted in FIG. 10.
A bright source protection feature is also provided in conventional night vision devices by decreasing the field voltage provided to the photocathode as a function of cathode current down to a predetermined threshold voltage commonly referred to as clamp voltage, a voltage level that is slightly greater than the minimum voltage required to allow photoelectrons to penetrate the ion barrier film that is deposited on the front face of the microchannel plate. This is accomplished through the use of a high value resistor between the cathode voltage multiplier in the power supply and the photocathode that creates a greater voltage drop under the high current conditions caused by a large number of photons incident on the photocathode (with a resulting high number of photoelectrons being provided by the photocathode). The photoelectrons provided by the photocathode represent a current flow increasing in magnitude with increasing light levels in the viewed field, such that the impedance of circuit element causes a decrease in the voltage level effective at the photocathode to move these electrons to the microchannel plate(s).
Recalling FIG. 9, it will be noted that this circuit architecture requires the use of two transformers, which are relatively large and heavy components of the circuit. Further, is seen that a typical conventional circuit architecture for a power supply of a night vision device provides a high-value resistor (generally 1-18 G-ohm) to the output of the photocathode voltage multiplier and a clamping circuit consisting of a voltage source and a low-leakage, high-voltage diode. As photocathode current flows through the high-value resistor, the photocathode voltage will decrease linearly until it reaches a voltage equal to the voltage source (plus the high-voltage low-leakage diode voltage drop). See FIG. 11 for a graphical illustration of this BSP voltage relationship at the photocathode. This voltage is commonly referred to as a clamp voltage, and is typically between 30 and 40 volts D.C.
The conventional method of BSP also has a disadvantage of decreased resolution for the I.sup.2 T. The reduced electrostatic field between the photocathode and the microchannel plate(s) input causes a reduced resolution for the tube. That is, photoelectrons liberated from the photocathode are not moved to the MCP as quickly under the reduced electrostatic field, this allows for lateral spreading of the photoelectrons and a loss of image definition. This is due to the fact that each photoelectron is emitted with some radial (or lateral) velocity component which is imparted to the electron during the photo-emission process. This radial velocity component causes the electron to move laterally away from the emission site at a constant rate which is independent of the magnitude of the electrostatic field between the photocathode and the microchannel plate. It can be readily appreciated, that the time required to transit the gap between the photocathode and microchannel plate will be increased under a reduced electrostatic filed. This increase in transit time allows for more lateral spreading and a commensurate reduction in resolution. Although this method of BSP serves to protect the photocathode from damage due to excess current densities, it will result in greatly reduced performance of the I.sup.2 T at high light levels (10.sup.-2 foot-candles and greater).